An integrated circuit may contain a high-voltage n-channel metal oxide semiconductor (MOS) power transistor which is configured to operate at a drain voltage which is significantly higher than an operating voltage for other transistors and circuits in the integrated circuit. For example, an integrated circuit which contains transistors and circuits which operate at 10 volts or less may also include a high-voltage n-channel MOS power transistor which operates at a drain voltage of over 300 volts and switches several amps. The body of the power transistor may be directly connected to the substrate of the integrated circuit, for example to provide a simpler fabrication process for the integrated circuit, compared to an integrated circuit with a high-voltage power transistor whose body is electrically isolated from the substrate. It may be desirable to determine if current through the power transistor is above a certain value when the power transistor is in the on state, without increasing the fabrication complexity of the integrated circuit or unduly increasing the size of the integrated circuit.